Trenches for underground cables and ditches and the like are commonly dug using hydraulically actuated excavator equipment having a bucket at a free end of an arm. When such a trench is being dug across a pipeline, special care must be taken to avoid damaging the pipeline in anyway due to contact from the excavator bucket. It is common practice to initially visually locate the pipeline by providing a vacuum type excavator which uses an injector to inject liquid into the ground. The liquid forms a slurry with the earth and a vacuum mechanism on the excavator then withdraws the slurry. A small hole from the surface to the underground pipeline in the order of a few feet in diameter is thus formed. Once the pipeline is visually located, the depth of the pipeline to the surface can be measured so that a bucket excavator can proceed with a better degree of assurance as to where the pipeline is located. Presently, excavator operators use the bucket excavator to dig the trench within a prescribed safe distance of the pipeline by measuring the depth of the trench relative to the surface. While digging, it is common for backfill to cover the pipeline so the operator of the excavator loses visual contact with the pipeline while digging with the bucket. Slight measurement errors when measuring the depth of the pipeline or the depth of the trench being formed by the excavator bucket can result in extremely costly and dangerous accidents causing damage to equipment and possible loss of life due to the extremely high pressures commonly found in pipelines.